I propose to study the genes responsible for the formation, structure, and recovery of the dauer larva of the free-living nematode, Caenorhabditis elegans. The dauer larva is a non-feeding, non-growing larval stage which is formed under conditions of starvation. It possesses a specialized impermeable cuticle and differs from all other larval stages in behavior and morphology. This "developmental switch" in the life cycle offers special technical advantages for genetic study. Genetic analysis of a complete catalogue of single-step mutants will be used to determine the structure of the genetic program for this discrete aspect of development. Light microscopy, electron microscopy, and biochemical analyses will be applied to the problem of how the genetic program is implemented. One class of mutants already characterized exhibits morphological alterations in sensory neurons, as determined by electron microscopy. Such mutants are useful for the study of nerve morphogenesis. A partial genetic pathway for dauer larva formation has already been established. Genetic characterization of additional mutants should reveal more details of this pathway. This work will provide a model for the genetic control of development in a simple animal.